Golf ball

ABSTRACT

A golf ball of the present invention includes a core and a cover located outside the core and having a plurality of dimples on a surface thereof A bottom of each of the dimples has a curved shape protruding toward an outside of the golf ball. A depth d of a center protruding portion of the bottom of the dimple is a perpendicular distance between a line S connecting both ends of an outer periphery of the dimple and a highest point of the protruding portion. A volume occupation ratio VR of the dimple is less than 0.75. A relationship between Shore D hardness H of a material of the cover and the depth d (unit: mm) satisfies the following Formula 1:(H-83)/(−300)&gt;d   (Formula 1).

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority from Japanese Patent Application No.2020-211280 filed Dec. 21, 2020, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball.

It is well known that when a golf ball is hit, backspin is applied tothe golf ball. If too much backspin is applied in a driver shot, theball tends to pop up. Accordingly, in order to extend a flight distance,it is generally required to reduce the amount of backspin. In order toreduce the amount of backspin, a golf ball which can have an increasedfrictional force with a face of a driver in a driver shot has beenproposed.

For example, JP 2017-006555 A discloses a golf ball having a pluralityof dimples on a surface thereof, of which the bottom has a curved shapeprotruding toward an outside of the golf ball. The golf ball satisfies apredetermined formula of a relationship among a deformation amount H ofthe golf ball when a predetermined load is applied to the golf ball, avirtual plane area S when no dimple exists on the surface of the golfball, and a pressurized area PS which is an area of the golf ballcontacting a plane when a predetermined load is applied to the golf ballsatisfies.

JP 2017-079905 A discloses a golf ball including a core, a cover, and atleast one intermediate layer therebetween, and satisfying a valueobtained by subtracting the surface hardness of an intermediatelayer-encased sphere from the surface hardness of the ball, and a valueobtained by subtracting the surface hardness of the core from thesurface hardness of the intermediate layer-encased sphere withinpredetermined ranges. Hardnesses at a core center, a position 5 mm fromthe core center, a position 10 mm from the core center, a position 15 mmfrom the core center, and a core surface, in core hardness distributionare within predetermined ranges. A value obtained by subtracting thecenter hardness of the core from the surface hardness of the core iswithin a predetermined range. A relationship V/H between a ball initialvelocity V and a golf ball deflection amount H when a predetermined loadis applied to the golf ball is within a predetermined range.

Furthermore, JP 2017-086579 A discloses a golf ball including atwo-layer core including an inner layer and an outer layer, a cover, atleast one intermediate layer between the core and the cover, and acoating film layer formed on the surface of the cover. In core hardnessdistribution, hardness Cc at the center of the inner layer core,hardness C10 at a position 10 mm from the center of the inner layercore, hardness Cs at the surface of the inner layer core, and hardnessCss at the surface of the outer layer core satisfy two predeterminedformulae. Furthermore, the sphere including the core encased by theintermediate layer has higher surface hardness than that of the ball.

SUMMARY OF THE INVENTION

Any of the documents of JP 2017-006555 A, JP 2017-079905 A, and JP2017-086579 A disclose that a dimple has a bottom with a curved shapeprotruding toward the outside of the golf ball. The present inventorshave found that the dimple may not necessarily contribute to an increasein an actual flight distance even if the protruding shape causesincreased frictional force to cause a decreased amount of backspin sincethe protruding shape is disadvantageous in terms of aerodynamicproperties. The frictional force of the dimple having a protrudingbottom with respect to the golf club is influenced by the materialhardness of the cover, but this is not particularly mentioned in theabove documents.

Then, an object of the present invention is to provide a golf ball inwhich a bottom of each of the dimples has a curved shape protrudingtoward an outside of the golf ball, which makes it possible to reliablyincrease flight distance.

In order to achieve the object, the present invention is a golf ballincluding a core and a cover located outside the core and having aplurality of dimples on a surface thereof, wherein a bottom of each ofthe dimples has a curved shape protruding toward an outside of the golfball. A depth d of a center protruding portion of the bottom of thedimple is a perpendicular distance between a line S connecting both endsof an outer periphery of the dimple and a highest point of theprotruding portion. A volume occupation ratio VR of the dimple is lessthan 0.75. A relationship between Shore D hardness H of a material ofthe cover and the depth d (unit: mm) satisfies the following formula 1:

(H-83)/(−300)>d   (Formula 1).

The Shore D hardness of the material of the cover may be 50 to 60.

The golf ball may further include an intermediate layer between the coreand the cover. In this case, Shore D hardness of a material of theintermediate layer may be 55 or more.

The Shore D hardness of the material of the intermediate layer may behigher than the Shore D hardness of the material of the cover.

The golf ball may further include a coating layer located outside thecover. In this case, the coating layer may contain delusteringparticles; average roughness Ra of a surface of the coating layer may be0.5 to 1.0; and the volume occupation ratio VR of the dimple may be lessthan 0.70.

The number of dimples having the depth d satisfying the formula 1 may be50% or more of the total number of dimples on a surface of the cover.

According to the present invention, when the bottom of the dimple has acurved shape protruding toward the outside of the golf ball, therelationship between the depth d of the protruding portion of the bottomof the dimple and the material hardness of the cover is specified as inthe above-mentioned Formula 1, whereby the contact area of the golf ballwith a club face in full shots (shots using a driver to a middle iron)can be increased, which makes it possible to reduce the amount ofbackspin. Furthermore, the curved protruding shape of the bottom of thedimple is disadvantageous in terms of aerodynamic properties. Onlyreduction in the amount of backspin makes it impossible to achieve anincrease in the flight distance of the golf ball, but the volumeoccupation ratio VR of the dimple is reduced to be less than 0.75 toraise the trajectory of the golf ball, whereby increase in the flightdistance can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view which illustrates an embodiment of a golfball according to the present invention;

FIG. 2 is an enlarged cross-sectional view of one dimple of the golfball illustrated in FIG. 1;

FIG. 3 is a perspective view which illustrates another embodiment of thegolf ball according to the present invention;

FIG. 4 is an enlarged cross-sectional view of one dimple of the golfball illustrated in FIG. 3; and

FIG. 5 is a perspective view which illustrates yet another embodiment ofthe golf ball according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of a golf ball according to the present invention will bedescribed below with reference to attached drawings. However, thepresent invention is not limited thereto.

The golf ball according to the present invention includes, as anembodiment thereof, a core (not shown) and a cover located outside thecore and having a plurality of dimples 10 on a surface thereof, asillustrated in FIG. 1. A portion of the surface of the golf ball 1located among the plurality of dimples 10 is usually referred to as aland portion 20. The land portion 20 constitutes the spherical surfaceof the golf ball 1. Accordingly, the land portion 20 has a surface withcurvature.

The planar shape of the dimple 10 formed on the surface of the golf ball1 (i.e., the shape recognized when an outer periphery 12 of the dimple10 or a boundary between the dimple 10 and the land portion 20 is viewedfrom immediately above the dimple) may be circular, polygonal,noncircular, and the like. In the present embodiment, the planar shapeis circular. In the case of the circular shape, the diameter of thedimple is preferably in a range of 2 to 5 mm. FIG. 5 illustrates anexample of a configuration in which the planar shape of the dimple isnoncircular. The dimple illustrated in FIG. 5 has a planar shapeincluding a combination of a plurality of curved portions (the number ofthe curved portions is 12 in the drawing) protruding toward an inside ofthe dimple.

The dimple 10 of the present embodiment has a shape in which a part ofthe bottom thereof is curved so as to protrude toward an outside of theball. FIG. 2 illustrates a cross-sectional view of the dimple 10 alongthe diameter thereof. As illustrated in FIG. 2, the dimple 10 has abottom 14 with a curved shape formed from one end to the other end ofthe outer periphery 12. The bottom 14 includes a portion with a curvedshape protruding toward the outside of the ball in a center region,i.e., a center protruding portion 15, and a portion with a curved shaperecessed from the outside of the ball in a ring-like region in its outerperiphery.

The bottom 14 is curved so that the depth thereof becomes the largest ata deepest point 18 located on both sides of the center protrudingportion 15. The location of the deepest point 18 on the plane ispreferably in a range of 20 to 45, more preferably in a range of 25 to40, and still more preferably in a range of 30 to 35, with a distancebetween the outer periphery 12 and a center point 16 of the dimple as100.

A depth d of the center protruding portion 15 of the dimple 10 is aperpendicular distance between a line S connecting both ends of theouter periphery 12 of the dimple and a highest point of the centerprotruding portion 15 (center point 16). A relationship between thedepth d (unit: mm) of the center protruding portion 15 and Shore Dhardness H of a material which forms a cover, to be described later,satisfies the following formula 1:

(H-83)/(−300)>d   (Formula 1).

That is, if the depth d is greater than the value of (H-83)/(−300), thecontact area of the golf ball with a face of the golf club is notsufficient, whereby frictional force cannot be increased. The value of(H-83)/(−300)-d is preferably 0.002 or more, and more preferably 0.005or more. The depth d providing such a value makes it possible toreliably provide an excellent frictional force depending on the materialhardness of the cover. The upper limit of the value of (H-83)/(−300)-dis not particularly limited, and it is preferably 0.010 or less, andmore preferably 0.008 or less.

The depth D of the dimple 10A differs according to the depth d of thecenter protruding portion 25. For example, the depth D of the dimple 10Ais greater than the depth d of the center protruding portion 25,preferably by 0.025 mm or more, and more preferably by 0.030 mm or more.The upper limit of the depth D of the dimple 10A is not particularlylimited, and it is preferably 0.200 mm or less, and more preferably0.150 mm or less.

For the curved shape of the bottom 24 of the dimple 10A, the lower limitof an edge angle A2 to the center protruding portion is preferably 2° ormore, and more preferably 3° or more. The upper limit of the edge angleA2 is preferably 15° or less, and more preferably 11° or less. The edgeangle A2 is an angle formed between a tangent line passing through apoint deeper than the depth d on the bottom curve by 10% and the line Smentioned above.

It is not necessary for all the dimples formed on the surface of thegolf ball to have the center-protruding shape described above.Preferably 50% or more, more preferably 70% or more, still morepreferably 80% or more, and most preferably 90% or more of all thedimples have the center-protruding shape. Of course, all the dimples mayhave the center-protruding shape. From the viewpoint of exertingexcellent aerodynamic isotropy and air resistance, it is preferable thatthe dimples having the center-protruding shape described above areuniformly arranged over the entire surface of the golf ball.

The upper limit of the total number of the dimples is, but not limitedto, preferably 500 or less, and more preferably 450 or less. The lowerlimit of the total number of the dimples is, but is not limited to,preferably 250 or more, and more preferably 300 or more.

A volume occupation ratio VR of the dimples (i.e., ratio of total volumeof the dimples formed in a portion downward from the plane surrounded bythe edge of the dimple in relation to a virtual spherical volume of thegolf ball obtained supposing that no dimple exists on the surface of thegolf ball) is less than 0.75%. The dimples having the center-protrudingshape described above are disadvantageous in terms of aerodynamicproperties. Therefore, the volume occupation ratio VR of the dimple isset to be less than 0.75% to raise the trajectory of the golf ball,whereby the flight distance can be increased. The volume occupationratio VR of the dimple is preferably 0.73% or less, and more preferably0.70% or less. The lower limit of the volume occupation ratio VR of thedimple is not particularly limited, and is, for example, preferably0.65% or more, and more preferably 0.68% or more.

A surface occupation ratio SR of the dimples (i.e., a ratio of the totalarea occupied by the dimples to the entire surface area of a virtualspherical surface of the golf ball obtained by supposing that no dimplesexist on the surface of the golf ball) is preferably 70% or more, morepreferably 75% or more, and still more preferably 80% or more. The upperlimit of the surface occupation ratio SR of the dimples is notparticularly limited, and is preferably 99% or less. It is particularlypreferable that at least three types of dimples with different sizes bearranged. This makes it possible to uniformly arrange the dimples on thespherical surface of the golf ball without a gap.

A material which forms the cover includes an ionomer resin, apolyurethane-based thermoplastic elastomer, a thermosettingpolyurethane, and a mixture thereof, but the material is not limitedthereto. In the cover, the abovementioned main component can be blendedwith other thermoplastic elastomers, polyisocyanate compounds, fattyacids or derivatives thereof, basic inorganic metal compounds, andfillers and the like.

The Shore D hardness H of the material which forms the cover satisfiesformula 1 as described above. Therefore, the Shore D hardness H of thematerial which forms the cover is dependent on the depth d of the centerprotruding portion of the dimple, and is, for example, preferably 50 ormore, and more preferably 53 or more. The Shore D hardness H of thematerial which forms the cover is preferably 65 or less, more preferably62 or less, and still more preferably 60 or less. Such numerical valueranges can provide an appropriate amount of spin in shots using a driverto a middle iron.

The lower limit of the thickness of the cover is preferably 0.2 mm ormore, and more preferably 0.4 mm or more, but the thickness of the coveris not limited thereto. The upper limit of the thickness of the cover ispreferably 4 mm or less, more preferably 3 mm or less, and still morepreferably 2 mm or less.

The core can be formed from a rubber composition containing rubber as amain component. As this rubber (base rubber) serving as the maincomponent, synthetic rubber and natural rubber can be widely used; andexamples of the rubber which can be used include polybutadiene rubber(BR), styrene butadiene rubber (SBR), natural rubber (NR), polyisoprenerubber (IR), polyurethane rubber (PU), butyl rubber (IIR), vinylpolybutadiene rubber (VBR), ethylene propylene rubber (EPDM), nitrilerubber (NBR) and silicone rubber, but the rubber is not limited thereto.As the polybutadiene rubber (BR), for example, 1,2-polybutadiene orcis-1,4-polybutadiene or the like can be used.

In the core, the rubber composition can be optionally blended with, forexample, a co-crosslinker, a crosslinking initiator, a filler, ananti-aging agent, an isomerizing agent, a peptizing agent, sulfur and anorganic sulfur compound, in addition to the abovementioned base rubber.In place of the rubber, a resin may be used as the main component, andfor example, a thermoplastic elastomer, an ionomer resin, or a mixturethereof can also be used.

Preferable examples of the co-crosslinker to be used includeα,β-unsaturated carboxylic acid or a metal salt thereof, but theco-crosslinker is not limited thereto. Examples of the α,β-unsaturatedcarboxylic acid or the metal salt thereof include: acrylic acid andmethacrylic acid; and zinc salts, magnesium salts and calcium saltsthereof. The blending ratio of the co-crosslinker is, but is not limitedto, for example, preferably approximately 5 parts by weight or more, andmore preferably approximately 10 parts by weight or more, with respectto 100 parts by weight of the base rubber. The blending ratio of theco-crosslinker is preferably approximately 70 parts by weight or less,and more preferably approximately 50 parts by weight or less.

As the crosslinking initiator, an organic peroxide is preferably used,and examples thereof include dicumyl peroxide, t-butyl peroxybenzoate,di-t-butyl peroxide, and1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, but the crosslinkinginitiator is not limited thereto. The blending ratio of the crosslinkinginitiator is, but is not limited to, preferably approximately 0.10 partsby weight or more, more preferably approximately 0.15 parts by weight ormore, and still more preferably approximately 0.30 parts by weight ormore, with respect to 100 parts by weight of the base rubber. Theblending ratio of the crosslinking initiator is preferably approximately8 parts by weight or less, and more preferably approximately 6 parts byweight or less.

Examples of the filler which can be used include silver, gold, cobalt,chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead,platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zincoxide and manganese oxide, but the filler is not limited thereto. Thefiller is preferably in the form of a powder. The blending ratio of thefiller is, but is not limited to, for example, preferably approximately1 part by weight or more, more preferably approximately 2 parts byweight or more, and still more preferably approximately 3 parts byweight or more, with respect to 100 parts by weight of the base rubber.The blending ratio of the filler is preferably approximately 100 partsby weight or less, more preferably approximately 80 parts by weight orless, and still more preferably approximately 70 parts by weight orless.

Examples of the anti-aging agent which can be used includecommercialized products such as NOCRAC NS-6 (manufactured by OuchiShinko Chemical Industrial Co., Ltd.), but the anti-aging agent is notlimited thereto. The blending ratio of the anti-aging agent is, but isnot limited to, preferably approximately 0.1 parts by weight or more,and more preferably approximately 0.15 parts by weight or more, withrespect to 100 parts by weight of the base rubber. The blending ratio ofthe anti-aging agent is preferably approximately 1.0 part by mass orless, and more preferably approximately 0.7 parts by mass or less.

The resilience of the core 40 can be improved by the addition of anorganic sulfur compound (peptizer). The organic sulfur compound isselected from thiophenols, thiocarboxylic acids, and metal saltsthereof. Examples of the thiophenols and the thiocarboxylic acidsinclude thiophenols such as pentachlorothiophenol,4-t-butyl-o-thiophenol, 4-t-butylthiophenol and 2-benzamidothiophenol,and thiocarboxylic acids such as thiobenzoic acid. As the metal saltsthereof, zinc salts and the like are preferable. The blending ratio ofthe organic sulfur compound is preferably approximately 0.5 parts byweight or more, and more preferably approximately 1 part by weight ormore, with respect to 100 parts by weight of the base rubber, but theratio is not limited thereto. The blending ratio of the organic sulfurcompound is preferably approximately 3 parts by weight or less, and morepreferably approximately 2 parts by weight or less.

The upper limit of the Shore D hardness of the material which forms thecore is preferably 60 or less, more preferably 50 or less, and stillmore preferably 40 or less. In addition, the lower limit of the Shore Dhardness of the material which forms the core is preferably 20 or more,and more preferably 30 or more, but the lower limit is not limitedthereto. With the material hardness of the core in such a range, thefeeling of a hit of the golf ball can be improved.

The lower limit of the thickness of the core may be 4.5 mm or more inorder to impart a predetermined repulsive force to the golf ball, and ismore preferably 10 mm or more. In addition, the upper limit of thethickness of the core is, but is not limited to, preferably 25 mm orless, and more preferably 20 mm or less. The core is not limited to acore formed of a single layer, and for example, the core may be formedof a plurality of layers. In this case, it is preferable that thehardness of each layer of the core be controlled so as to increase fromthe inside to the outside of the golf ball.

An intermediate layer (not shown) may be optionally provided between thecore and the cover. The intermediate layer is provided and can providean appropriate amount of spin in shots using a driver to a middle iron.

A material which is preferably used as the main material of theintermediate layer is the following heated mixture, but the material isnot limited thereto. This material is used for the intermediate layer,whereby the amount of spin can be decreased at the time of hitting, anda long flight distance can be obtained. The mixture contains: a baseresin in which (a) a binary random copolymer of olefin-unsaturatedcarboxylic acid, and/or a metal ion neutralized product of a binaryrandom copolymer of olefin-unsaturated carboxylic acid, and (b) aternary random copolymer of olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester, and/or a metal ion neutralizedproduct of a ternary random copolymer of olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester are blended so that the weightratio of 100:0 to 0:100 is achieved; (e) a non-ionomeric thermoplasticelastomer which is blended so that the weight ratio of 100:0 to 50:50 isachieved with respect to the base resin; (c) 5 to 150 parts by weight ofa fatty acid having a molecular weight of 228 to 1500 and/or aderivative thereof, with respect to 100 parts by weight of a resincomponent containing the base resin and the component (e); and (d) 0.1to 17 parts by weight of a basic inorganic metal compound which canneutralize an unneutralized acid group in the base resin and thecomponent (c).

Here, the “main material” means a material which accounts for 50% byweight or more, preferably 60% by weight or more, and still morepreferably 70% by weight or more, of the total weight of theintermediate layer.

The Shore D hardness of the material which forms the intermediate layeris preferably 55 or more, and more preferably 57 or more. The Shore Dhardness of the material which forms the intermediate layer ispreferably higher than the Shore D hardness of the material which formsthe cover. This can provide an appropriate amount of spin in shots usinga driver to a middle iron. The upper limit of the Shore D hardness ofthe material which forms the intermediate layer is preferably 65 orless, and more preferably 63 or less, but the upper limit is not limitedthereto.

The thickness of the intermediate layer is, but is not limited to,preferably 0.5 mm or more, and more preferably 1 mm or more. Thethickness of the intermediate layer 20 is preferably 10 mm or less, morepreferably 5 mm or less, and still more preferably 3 mm or less.

On the surface of the cover, a coating layer (also referred to as apainted layer) (not shown) may be optionally provided. The coating layeris formed of a coating material composition. The coating materialcomposition may contain delustering particles. The coating materialcomposition is not particularly limited, and for example, aurethane-based coating material is preferably used. Given the need to becapable of enduring the severe use environment of the golf ball, atwo-part curable urethane coating material is preferable, with the useof a non-yellowing urethane coating material being particularlypreferable.

In the case of the two-part curable urethane coating material, it ispreferable to use, as the main agent, various polyols such as saturatedpolyester polyols, acrylic polyols and polycarbonate polyols. It ispreferable to use, as an isocyanate which is a curing agent, anon-yellowing polyisocyanate, examples of which include hexamethylenediisocyanate, isophorone diisocyanate, and adducts, biurets,isocyanurates, or mixtures thereof, of hydrogenated xylylenediisocyanate.

Examples of the delustering particles include silica particles, melamineparticles and acrylic particles. Specific examples include silicaparticles, polymethyl methacrylate particles, polybutyl methacrylateparticles, polystyrene particles and polybutyl acrylate particles.Either organic particles or inorganic particles may be used, with theuse of silica particles being particularly suitable.

If such delustering particles are contained in the coating layer, theaerodynamic performance of the golf ball is disadvantageous, but in thepresent embodiment, the volume occupation ratio VR of the dimple is lessthan 0.75, to raise the trajectory, whereby the flight distance can bemaintained. When the delustering particles are contained in the coatinglayer, the volume occupation ratio VR of the dimple may be less than0.70. Thereby, the trajectory of the golf ball is further raised,whereby the flight distance can be increased.

In terms of the light-quenching properties and the coating properties,the delustering particles have a BET specific surface area, which ispreferably 200 to 400 m²/g, and more preferably 250 to 350 m²/g. Interms of the spin performance and the light-quenching properties, thedelustering particles have an average primary particle size which ispreferably 1.0 to 3.0 μm, and more preferably 2.0 to 2.8 μm. When theaverage primary particle size is more than 3.0 μm, the ball surfacebecomes rough, which may have an adverse effect on the spin performanceof the golf ball, reducing the spin performance. In addition, when theaverage primary particle size is too small, the light-quenching effectmay diminish.

The blending amount of the delustering particles per 100 parts by massof the main agent (the total amount of resin components and solvent) inthe coating material composition of the coating layer may be set to bepreferably 5 to 10 parts by mass. When this blending amount is too high,the viscosity of the coating material composition increases and thecoating operation tends to be poor. When it is too low, thelight-quenching effect may diminish. The coating layer has averagesurface roughness Ra which, from the standpoint of both the amount ofspin of the ball on approach shots and the light-quenching properties,is suitably 0.5 to 1.0. The surface roughness Ra of the coating filmmeans the arithmetic average roughness in JIS B0601 (1994).

The shape of the dimple of the golf ball of the present embodiment hasbeen described with reference to FIGS. 1 and 2, but the presentinvention is not limited to a dimple having such a shape. For example, adimple having a shape illustrated in another embodiment illustrated inFIGS. 3 and 4 may be used. Hereinafter, the other embodiment will bedescribed.

As illustrated in FIG. 3, on the surface of a golf ball 1, a pluralityof dimples 10A are formed. In the dimple 10A, the bottom thereof has acurved shape protruding toward the outside of the ball in the center ofthe dimple, as illustrated in FIG. 4. However, as illustrated in FIG. 2,the bottom is not entirely curved and the leading edge portion has aplanar shape.

More specifically, a bottom 24 shaped from one end of an outer periphery22 of the dimple 10A to the other end has a portion with a curved shapeprotruding toward the outside of the ball in a center region thereof anda flat shape in a further central portion thereof, i.e., a centerprotruding portion 25, and a portion with a curved shape recessed fromthe outside of the ball in a ring-like region in its outer periphery.

In the flat region of the center protruding portion 25, a distance Wbetween both ends 27 is preferably in a range of 35 to 65, morepreferably in a range of 40 to 60, and still more preferably in a rangeof 45 to 55, with a distance between the outer periphery 22 and a centerpoint 26 of the dimple as 100.

An outer periphery 29 of the flat region of the center protrudingportion 25 is configured so that a corner portion thereof is chamfered.The corner portion is chamfered, whereby the outer periphery 29 caneffectively contribute to the increase in the contact area of thepresent invention, and as a result, the spin performance can beimproved. The radius of curvature R of the chamfered corner portion ispreferably 0.4 mm or more, and more preferably 0.5 mm or more. The upperlimit of the radius of curvature R is preferably 2.5 mm or less, andmore preferably 2.0 mm or less.

A depth d of the center protruding portion 25 in the flat region isconstant. The depth d of the center protruding portion 25 is determinedon the basis of the line S connecting both ends of the outer periphery22 of the dimple as the reference, as described above. The relationshipbetween the depth d of the center protruding portion 25 and the Shore Dhardness H of the material which forms the cover satisfies the aboveformula 1 as with the embodiment of FIG. 2. The value of (H-83)/(−300)-dis preferably 0.002 or more, and more preferably 0.005 or more. Thedepth d providing the value can certainly provide an excellentfrictional force depending on the material hardness of the cover. Theupper limit of the value of (H-83)/(−300)-d is not particularly limited,and is preferably 0.010 or less, and more preferably 0.008 or less.

In regions on both sides of the center protruding portion 25, the bottomof the dimple is curved so that the depth thereof becomes greatest atdeepest point 28. The location of the deepest point 28 on the plane ispreferably in a range of 25 to 55, more preferably in a range of 30 to50, and still more preferably in a range of 35 to 45, with the distancebetween the outer periphery 22 and the center point 26 of the dimple as100.

EXAMPLES

A golf ball having a configuration shown in Example 1 of Table 1 wasproduced. Table 3 shows the blending of a core. Table 4 shows theblendings of an intermediate layer and the cover. Table 5 shows theblending of a paint layer. The protruding shape of a dimple was a shapehaving a flat region in a center protruding portion as illustrated inFIG. 4. The dimples were arranged to have a pattern illustrated in FIG.5.

TABLE 1 Examples 1 2 3 4 5 Intermediate Blending A A A A B layerMaterial hardness 57 57 57 57 51 Cover Blending C D E C C Materialhardness H 55 59 62 55 55 Paint layer Blending F F F G F Delusteringparticles Absent Absent Absent Present Absent Dimples Number 326 326 326326 326 VR 0.70 0.70 0.70 0.67 0.70 Protruding Number 318 318 318 318318 Depth d 0.085 0.075 0.068 0.085 0.085 [mm] Value of (H-83)/(−300)0.093 0.080 0.070 0.093 0.093 Value of (H-83)/(−300)-d 0.008 0.005 0.0020.008 0.008 Evaluation of Formula 1 Satisfactory SatisfactorySatisfactory Satisfactory Satisfactory Ratio of protruding dimples [%]98 98 98 98 98 Amount of spin 2736 2712 2688 2738 2745 (driver) [rpm]Evaluation Good Good Good Good Good Flight distance (driver) [m] 232.4233.5 235.2 232.1 232.2 Evaluation Good Good Good Good Good Amount ofspin 5093 5221 5324 5131 5630 (middle iron) [rpm] Evaluation Good GoodGood Good Poor

TABLE 2 Comparative Examples 1 2 3 4 5 Intermediate Blending A A A A Alayer Material hardness 57 57 57 57 57 Cover Blending C D E C C Materialhardness H 55 59 62 55 55 Paint layer Blending F F F F G Delusteringparticles Absent Absent Absent Absent Present Dimples Number 326 326 326326 326 VR 0.70 0.70 0.70 0.78 0.75 Protruding Number 318 318 318 318318 Depth d 0.102 0.085 0.080 0.085 0.085 [mm] Value of (H-83)/(−300)0.093 0.080 0.070 0.093 0.093 Value of (H-83)/(−300)-d −0.009 −0.005−0.010 0.008 0.008 Evaluation of formula 1 Not Not Not SatisfactorySatisfactory satisfactory satisfactory satisfactory Ratio of protrudingdimples [%] 98 98 98 98 98 Amount of spin 2806 2784 2748 2741 2742(driver) [rpm] Evaluation Bad Bad Poor Good Good Flight distance 230.2230.5 230.8 228.8 228.5 (driver) [m] Evaluation Bad Bad Bad Bad BadAmount of spin 5225 5315 5390 5087 5122 (middle iron) [rpm] EvaluationPoor Poor Poor Good Good

TABLE 3 Polybutadiene 100 Acrylic acid zinc 21 Organic peroxide A 0.3Organic peroxide B 0.3 Anti-aging agent 0.1 Zinc oxide 29.4Pentachlorothiophenol zinc salt 0.6

Polybutadiene in Table 3 is “BR01” (trade name) manufactured by JSRCorporation. Acrylic acid zinc is manufactured by NIPPON SHOKUBAI CO.,LTD. An organic peroxide A is “PERCUMYL D” (trade name) manufactured byNOF CORPORATION. An organic peroxide B is “PEROXA 40” (trade name)manufactured by NOF CORPORATION. An anti-aging agent is2,2-methylenebis(4-methyl-6-butylphenol) with “Nocrac NS-6” (trade name)manufactured by OUCHI SHINKO CHEMICAL INDUSTRIAL CO., LTD. Zinc oxide is“ZINC OXIDE 3 TYPES” (trade name) manufactured by SAKAI CHEMICALINDUSTRY CO., LTD. A pentachlorothiophenol zinc salt is manufactured byZhejiang Cho & Fu Chemical Co., Ltd.

TABLE 4 A B c D E HPF1000 56 100 — — — Himilan 1605 44 — 50 50 50 AM7329— — — 15 50 Surlyn 9320 — — 50 35 — Titanium oxide — —  4 4  4

“HPF1000” in Table 4 is an ionomer resin manufactured by THE DOWCHEMICAL COMPANY. “Himilan 1605” is an ionomer resin manufactured byDuPont-Mitsui Polychemicals Co., Ltd. “AM7329” is an ionomer resinmanufactured by DuPont-Mitsui Polychemicals Co., Ltd. “Surlyn 9320” isan ionomer resin manufactured by THE DOW CHEMICAL COMPANY.

TABLE 5 F G Main agent Polyol 29.84 29.84 Delustering 0 9 particlesSolvent 70.16 70.16 Curing agent Isocyanate 42 42 Solvent 58 58

As a “polyol” of a main agent in Table 5, a polyester polyol synthesizedby the following method was used. First, a reactor equipped with areflux condenser, a dropping funnel, a gas inlet and a thermometer wascharged with 140 parts by mass of trimethylolpropane, 95 parts by massof ethylene glycol, 157 parts by mass of adipic acid and 58 parts bymass of 1,4-cyclohexanedimethanol, followed by raising the temperatureto 200 to 240° C. while stirring, for heating (reacting) for 5 hours.Then, a polyester polyol was obtained, which had an acid value of 4, ahydroxyl value of 170 and a weight-average molecular weight (Mw) of28,000. As “delustering particles”, “Finesil X-35” manufactured by MaruoCalcium Co., Ltd. was used.

As an “isocyanate” for a curing agent, Duranate TPA-100 (trade name)manufactured by Asahi Kasei Corporation as a nurate body (isocyanuratebody) of hexamethylene diisocyanate (HMDI) (NCO content: 23.1%,non-volatile content: 100%) was used. Butyl acetate was used as asolvent for both a main agent and a curing agent. The two-part curableurethane coating material having the above blending was applied with anair spray gun onto the surface of a cover on which dimples had beenformed, to form a paint layer.

The golf ball of Example 1 having such a configuration was subjected totests to evaluate the amount of spin and flight distance of the golfball. First, a driver club (“TourB XD-5” (W# 1) (loft angle:)9.5°manufactured by Bridgestone Sports Co., Ltd.) was mounted to a golf ballhitting robot, and a golf ball as a sample was hit at a head speed of 45m/s to measure the amount of backspin and the flight distance. A middleiron club (“TourB X-CB” (I# 6) manufactured by Bridgestone Sports Co.,Ltd.) was mounted to a golf ball hitting robot, and a golf ball as asample was hit at a head speed of 42 m/s to measure the amount ofbackspin. The results are shown in Table 1.

For comparison, golf balls having configurations of Comparative Examples2 to 4 shown in Table 2 were produced. The golf balls were subjected totests to evaluate the amounts of spin and flight distances of the golfballs in the same manner as in Example 1.

As shown in Table 1, the golf ball of Example 1 having a dimple volumeoccupation ratio VR of less than 0.75 and satisfying the condition of(H-83)/(−300) >d of formula 1 had a less amount of backspin in a drivershot than that of Comparative Examples 2 and 3 not satisfying thecondition of Formula 1, which could have an increased flight distance.The golf ball of Example 1 also had a less amount of backspin in themiddle iron than that of Comparative Examples 2 and 3. This is alsoconsidered to make it possible to increase the flight distance using themiddle iron.

Furthermore, the golf ball of Example 1 had substantially the sameamount of backspin in the driver shot as that of Comparative Example 4satisfying the condition of formula 1 and having a dimple volumeoccupation ratio VR of 0.75 or more, but the golf ball of Example 1having a volume occupation ratio VR of 0.70 could have an increasedflight distance. The golf ball of Example 1 had substantially the sameamount of backspin in the middle iron as that of Comparative Example 4.This is also considered to make it possible to increase the flightdistance using the middle iron.

Table 2 shows the golf ball of Comparative Example 1 having the samematerial hardness of the cover as that of Example 1 and not satisfyingthe condition of formula 1. In a simulation considering the above testresults, in Comparative Example 1, the depth d of the protruding dimplewas too large, as shown in Table 2, whereby the contact area of the golfball with the club was small, and the golf ball of Comparative Example 1had an amount of backspin more than, and a flight distance less than,those of Example 1. The golf ball of Comparative Example 1 is alsoconsidered to have a greater amount of backspin using the middle ironthan that in Example 1, which has a decreased flight distance.

Table 1 shows the golf balls of Examples 2 and 3 having the samematerial hardness of the cover as that of Comparative Examples 2 and 3and satisfying the conditions of Formula 1. In a simulation consideringthe above test results, in Examples 2 and 3, the depth d of theprotruding dimple was smaller than that of Example 1, as shown in Table1, whereby the contact area of the golf ball with the club was large,and the golf balls of Examples 2 and 3 can have a smaller amount ofbackspin than that of Example 1, and therefore can have an increasedflight distance. It is considered that the golf balls of Examples 2 and3 can have a smaller amount of backspin using the middle iron than thatin Comparative Examples 2 and 3, which can have an increased flightdistance.

Furthermore, as shown in Table 1, the golf ball of Example 4 havingalmost the same configuration as that of Example 1 except thatdelustering particles were blended with a paint layer was produced, andsubjected to tests to evaluate the amount of spin and the flightdistance as with Example 1. For comparison, as shown in Table 2, thegolf ball of Comparative Example 5 satisfying the condition of Formula1, having a dimple volume occupation ratio VR of 0.75 or more, andincluding a paint layer blended with delustering particles was produced,and similarly subjected to the tests. As a result, the amounts ofbackspin using the driver shot in Example 4 and Comparative Example 5were almost equal to each other, but the golf ball of Example 4 having avolume occupation ratio VR of 0.67 could have an increased flightdistance. The amount of backspin using the middle iron in Example 4 wasalso almost equal to that in Comparative Example 5. Therefore, it isconsidered that the flight distance using the middle iron can also beincreased.

Table 1 shows the golf ball of Example 5 having almost the sameconfiguration as that of Example 1 except that the material hardness ofthe intermediate layer is lower than the material hardness of the cover.In a simulation considering the above test results, as shown in Table 1,the amount of backspin in the driver shot in Example 5 is equal to thatin Example 1, and the flight distance can be maintained, but the amountof backspin in the middle iron was increased. Therefore, the increase inthe flight distance using the middle iron is considered to be poor.

What is claimed is:
 1. A golf ball comprising: a core; and a coverlocated outside the core and having a plurality of dimples on a surfacethereof, wherein a bottom of each of the dimples has a curved shapeprotruding toward an outside of the golf ball; a depth d of a centerprotruding portion of the bottom of the dimple is a perpendiculardistance between a line S connecting both ends of an outer periphery ofthe dimple and a highest point of the protruding portion, a volumeoccupation ratio VR of the dimple is less than 0.75, and a relationshipbetween Shore D hardness H of a material of the cover and the depth d(unit: mm) satisfies the following Formula 1:(H-83)/(−300)>d   (Formula 1).
 2. The golf ball according to claim 1,wherein the Shore D hardness of the material of the cover is 50 to 60.3. The golf ball according to claim 1, further comprising anintermediate layer between the core and the cover, wherein Shore Dhardness of a material of the intermediate layer is 55 or more.
 4. Thegolf ball according to claim 3, wherein the Shore D hardness of thematerial of the intermediate layer is higher than the Shore D hardnessof the material of the cover.
 5. The golf ball according to claim 1,further comprising a coating layer located outside the cover, whereinthe coating layer contains delustering particles, average roughness Raof a surface of the coating layer is 0.5 to 1.0, and the volumeoccupation ratio VR of the dimple is less than 0.70.
 6. The golf ballaccording to claim 1, wherein the number of dimples having the depth dsatisfying the Formula 1 is 50% or more of the total number of dimpleson a surface of the cover.